Dual frequency radio tag for a radio frequency identification system

ABSTRACT

A dual frequency radio tag for a radio frequency identification system is provided that includes an antenna system and a first receiver coupled to the antenna system. The first receiver receives and processes request signals from a first unit type at a first frequency. A second receiver is coupled to the antenna system and receives and processes request signals from a second unit type at a disparate second frequency. A transmitter is coupled to the antenna system and transmits response signals from the radio tag at the second frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/357,435, filed Jul. 20, 1999, by Wayne E. Steeves entitled “RadioFrequency Identification System and Method”.

This application is related to U.S. application Ser. No. 11/___,___,filed Nov. 9, 2005, by Wayne E. Steeves entitled “Method and System forNetworking Radio Tags in a Radio Frequency Identification System”.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to asset tracking and control and moreparticularly to a dual frequency radio tag for a radio frequencyidentification system.

BACKGROUND OF THE INVENTION

The management and tracking of personnel, assets, and other objects isrequired in a wide variety of environments and is often cumbersome,labor intensive, and expensive. Radio receivers and transmitters havebeen used for many years to identify personnel and objects in suchenvironments. For example, many systems are known for attaching radiotags to items, such as automobiles, so that when automobiles equippedwith radio tags enter a certain area, such as a toll booth area, theautomobiles are automatically identified and the appropriate tolls arededucted from corresponding accounts, thereby obviating the need fordrivers to stop and make payment at toll booths. Innumerable otherapplications for such radio tag systems have been identified, in areasranging from inventory control to facility security to sporting eventtiming.

Radio frequency identification (RFID) systems generally use a fixedposition base station capable of reading portable tags attached topersonnel, assets, or other objects. Typical base stations include anantenna, a reader, and a computer. If an RFID system covers a largeenough area, multiple base stations may be necessary to provide adequatecoverage for the large area.

In addition, bandwidth use is high because the base station computercommunicates with and processes information from every radio tag withinits operational range. The lack of available bandwidth limits multi-readcapabilities of the system which allows a base station to interact withmore than one radio tag at any particular time.

SUMMARY OF THE INVENTION

In accordance with the present invention, an improved dual frequencyradio tag for a radio frequency identification system is provided whichsubstantially eliminates or reduces disadvantages and problemsassociated with previous radio frequency identification systems.

According to one embodiment of the present invention, there is provideda dual frequency radio tag for a radio frequency identification systemcomprising an antenna system and a first receiver coupled to the antennasystem. The first receiver receives and processes request signals from afirst unit type at a first frequency. A second receiver is coupled tothe antenna system and receives and processes request signals from asecond unit type at a disparate second frequency. A transmitter iscoupled to the antenna system and transmits response signals at thesecond frequency.

More specifically, in accordance with one embodiment of the presentinvention, the first unit type is a base station and the second unittype is another tag. In this and other embodiments, a processor isfurther coupled to the first receiver, the second receiver, and thetransmitter to generate response signals for transmission over thetransmitter. The response signals are generated in response to theprocessor receiving request signals from the first receiver or thesecond receiver.

Technical advantages of the present invention include providing animproved radio tag. In particular, a radio tag communicates at multipleradio frequencies. A first radio frequency is used for communicationsfrom a base station and a second radio frequency is used forcommunications from the radio tag.

Other technical advantages will be readily apparent to one skilled inthe art from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numbers represent like parts, and in which:

FIG. 1 is a block diagram illustrating a radio frequency identification(RFID) system in accordance with one embodiment of the presentinvention;

FIG. 2 is a schematic block diagram illustrating a multi-frequencywakeup radio tag for use in the RFID system of FIG. 1 in accordance withone embodiment of the present invention;

FIG. 3 is a schematic block diagram illustrating a high sensitivityreceiver for the radio tag of FIG. 2 in accordance with one embodimentof the present invention;

FIG. 4 is a block diagram illustrating a low power receiver for theradio tag of FIG. 2 in accordance with one embodiment of the presentinvention;

FIG. 5 is a timing diagram illustrating first and second stage quenchfrequency oscillator outputs for the low power receiver of FIG. 4 inaccordance with one embodiment of the present invention;

FIG. 6 is a flow diagram illustrating the operation of the low powerreceiver of FIG. 4 in low and full power modes;

FIG. 7 is a block diagram illustrating an asset tracking system usingthe radio tag of FIG. 2 to form a tag network in accordance with oneembodiment of the present invention;

FIG. 8 is a flow diagram illustrating the operation of a radio tag inthe asset tracking system of FIG. 7 in accordance with one embodiment ofthe present invention;

FIG. 9 is a block diagram illustrating a retail facility using adistributed asset control system in accordance with one embodiment ofthe present invention;

FIG. 10 is a flow diagram illustrating operation of the distributedasset control system in the retail facility of FIG. 9 in accordance withone embodiment of the present invention;

FIG. 11 is a block diagram illustrating a secure facility using adistributed asset control system in accordance with another embodimentof the present invention;

FIG. 12 is a flow diagram illustrating operation of the distributedasset control system in the secure facility of FIG. 11 in accordancewith one embodiment of the present invention; and

FIG. 13 is a block diagram illustrating a shipping facility using adistributed asset control system in accordance with still anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a radio frequency identification (RFID) system 10 inaccordance with one embodiment of the present invention. The RFID system10 is a system used to track and identify objects or persons byattaching a transponder, or radio tag, to each object or person beingtracked. The RFID system 10 communicates at one or more wirelessfrequencies including low frequencies (LF), very low frequencies (VLF),very high frequencies (VHF), ultra-high frequencies (UHF), microwaves,or other suitable frequencies.

Referring to FIG. 1, the RFID system 10 includes a base station 11 thatresides at a fixed location and communicates with one or more radio tags20 by an analog signal at a specified radio frequency. Radio tag 20 is aremote, portable, self-contained device that may be affixed to amoveable item, such as a person, inventory, or a vehicle.

The base station 11 is a conventional unit and includes a reader 12, acontrol system 14, and a base station antenna 16. Control system 14 maybe implemented as a mainframe or other stand alone computer, server,personal computer, or any other type of computing device capable ofcontrolling operation of the base station 11. Base station antenna 16transmits and receives signals on various radio frequencies as necessaryto provide communications between base station 11 and radio tags 20.Reader 12 acquires incoming signals from base station antenna 16 anddemodulates the incoming signal for processing by control system 14.Reader 12 also modulates signals generated by control system 14 onto acarrier wave and transmits the modulated signal through base stationantenna 16 as an analog communicated signal.

Signals are used within RFID system 10 to transmit messages between basestation 11 and radio tags 20. Communicated signals destined for aspecific radio tag 20 or the base station 11 are referred to as explicitcommunications since a specific destination device is referenced in themessage. Other messages may be between base station 11 and all radiotags within RFID system 10. These communications are referred to asnon-explicit, or global, communications since the message is notdirected at a specific radio tag and requires a response or othersuitable action from each radio tag receiving the message.

In one embodiment, analog communicated signals contain a message thatrequests radio tag 20 to perform some action such as responding to aquery, forwarding a message, or initiating a query of surrounding radiotags 20. The analog communicated signals also contain a wakeup signaturethat precedes each message and informs radio tag 20 to prepare toreceive an incoming message.

In accordance with one aspect of the present invention, each radio tag20 includes logic and circuitry for retransmission of signals receivedfrom base station 11 and other radio tags 20. As used herein, the termeach means every one of at least a subset of the identified items. Eachradio tag 20 has an associated operational range that defines the areain which signals transmitted by radio tag 20 may be received by otherdevices such as radio tag 20 and base station 11. If radio tag 20receives an explicit communication from base station 11 that identifiesthat radio tag 20 as the destination, radio tag 20 will process themessage and transmit a response to base station 11. If another radio tag20 is the destination for the explicit communication, the receivingradio tag 20 will retransmit the received message for reception by thedestination radio tag 20.

If radio tag 20 receives a non-explicit communication from base station11, radio tag 20 will process the received message, transmit a responseto base station 11, and retransmit the message to other radio tags 20within the operational range of radio tag 20.

The present invention uses slot allocation as taught by U.S. patentapplication Ser. No. 08/789,148 entitled Radio Tag System and Methodwith Improved Tag Interference Avoidance, incorporated herein byreference, to facilitate the transmission and retransmission of messageswithin RFID system 10.

In accordance with the retransmission capability of radio tag 20, theone or more radio tags 20 form a tag network 22 in which a radio tag 20will retransmit a received message which is destined for anotheraddress. Each device, such as a radio tag 20 or base station 11, has aunique address within the RFID system 10. The destination address couldbe another radio tag 20 or base station 11. Since radio tags 20 areoperable to retransmit received signals, radio tags 20 outside theoperational range of base station 11 may communicate with base station11 through other radio tags 20. The retransmission capability of radiotags 20 increases the operational range of both base station 11 andradio tags 20 so that additional base stations are not necessary toprovide adequate coverage for RFID system 10. As a result, some radiotags 20 in RFID system 10 may exist outside the operational range ofbase station 11 and still function as part of RFID system 10. Radio tags20 outside the operational range of the base station 11 transmitresponse messages to the base station 11 through other radio tags 20closer in proximity to the base station 11.

In a particular embodiment, radio tags 20 are active radio tags thatcontain a local power source. As described in more detail below inconnection with FIG. 2, active radio tags 20 may have internalprocessing capability to determine an appropriate response to anincoming message. Radio tags 20 may then transmit the response at a highpower thereby increasing the operational range of the radio tag ascompared to passive radio tags which do not contain a local powersource.

Passive radio tags utilize the energy from an incoming message tomodulate a fixed response onto the incoming signal and forward thatsignal with the modulated response back to the sending device. Theoperational range of passive tags is necessarily limited due to the lackof a local power source.

In accordance with another embodiment of the present invention, radiotag 20 may be a primary tag 20 assigned responsibility for one or moresecondary tags 22 and 24. In accordance with this aspect of theinvention, the primary tag 20 polls the secondary tags 22 and 24 todetermine a status of monitored assets and reports the status orirregularities to the base station 11. Polling may be performedperiodically on a timer basis or in response to a request from the basestation 11 or other suitable device. In a particular embodiment, theprimary radio tag 20 may be an active radio tag as previously describedand the secondary tag 22 may be a passive tag.

Primary tags 20 may perform distributed processing. The distributedprocessing capability of the present invention reduces processingresponsibility of base station 11 and increases available bandwidth inthe RFID system 10 since fewer messages are transmitted, received, andprocessed by the base station 11. FIG. 2 illustrates a radio tag 29 foruse in the RFID system 10 of FIG. 1 in accordance with one embodiment ofthe present invention. In this embodiment, the radio tag 29 is a dualfrequency radio tag that can be employed in a tag network 22 or as aprimary tag. Although radio tag 29 will be described as a dual frequencyradio tag, radio tag 29 may be generally described as a multiplefrequency radio tag incorporating transmit and receive capabilities forone or more frequencies. It will be understood that other suitable radiotags capable of communication with other radio tags may be used in theRFID system 10.

The dual frequency radio tag 29 allows the radio tag 29 to communicatewith the base unit at a first frequency and other tags at a secondfrequency. The use of dual frequencies allows the RFID system 10 toutilize a shorter range, more controllable frequency for communicationsbetween base station 11 and a set of radio tags 20 within itsoperational range. Since radio tags 20 should minimize powerconsumption, the second frequency provides broader coverage at low powerlevels as compared to the first frequency. The use of dual frequenciesalso allows radio tag 20 to determine whether a received signal camefrom base station 11 or other radio tags 20 without additionalprocessing. Dual frequency as used herein refers to two or morefrequencies utilized by radio tag 29. Thus, radio tag 29 may transmitsignals on one or more frequencies and received signals on at least twofrequencies.

The frequency spectrum includes three general ranges of frequenciessuitable for RFID system applications. These ranges include kilohertzfrequencies on the low end of the spectrum up to gigahertz frequenciesat the high end of the spectrum. At the low end of the spectrum are verylow frequencies (VLF) and low frequencies (LF). The VLF/LF frequencieshave limited range, but signals transmitted on these frequencies arevery controllable. Thus, they are particularly useful for applicationsrequiring controlled transmission of signals to a specific geographicarea. An example of an application for VLF/LF frequencies are providingwakeup signals to radio tags as they enter a specific area. The VLF/LFfrequencies are generally not suitable for transmitting signals back tothe base station since radio tags have insufficient power to overcomenoise and other interference present in these frequency ranges.

The middle of the frequency spectrum includes very high frequencies(VHF) and ultra-high frequencies (UHF). These frequencies arecharacterized by low noise and reliable transmission. However, VHF/UHFfrequencies cannot easily be directionally controlled. In addition, itis difficult to control range. Thus, these frequencies transmit in alldirections. VHF/UHF frequencies are best suited for radio tag responsessince the orientation between the base station 11 and radio tag 20 isirrelevant and signals may be transmitted at very low power since thereis low noise present in these frequency ranges.

At the upper end of the frequency spectrum are microwaves. Thesefrequencies can be made extremely directional and are very sensitive toenvironmental interference. Microwaves generally require a direct lineof sight between transmitter and receiver. In general, microwavefrequencies have limited application due to their extreme sensitivity toenvironmental interference. A specific application for microwavefrequencies is an RFID system for a toll booth. Since microwaves aredirectional and can be focused, the base station 11 can transmit signalsto a specific area where a vehicle, and thus its radio tags 20, willenter as it proceeds through the toll booth.

Referring to FIG. 2, dual frequency radio tag 29 includes an internalantenna system 38, a transmitter 32, a first frequency receiver 34, asecond frequency receiver 36, a processor 30, and a local power source40.

Internal antenna system 38 allows dual frequency radio tag 29 totransmit and receive radio frequency signals at specified frequencies.In one embodiment, the internal antenna system 38 is a dual antennaassembly. In another embodiment in which the frequencies are closelyspaced, internal antenna system 38 is implemented using a conventionalantenna with conventional switching circuitry allowing use of internalantenna system 38 with either transmitter 32, first frequency receiver34, or second frequency receiver 36.

First frequency receiver 34 and second frequency receiver 36 eachinclude a tuner circuit to receive and filter the signals at a specifiedfrequency. The tuned and filtered signal is then demodulated creating adigital signal suitable for processing by processor 30. Processor 30 maybe any suitable general purpose processor, microprocessor, ormicrocontroller. Processor 30 includes a processor unit, memory, andstorage.

Local power source 40 may be a battery, solar cell system, or othersuitable portable power source. In an exemplary embodiment, local powersource 40 is a three volt lithium coin cell battery. The characteristicof a battery or power source connected to electrical components is aleakage of a certain quantity of electrical current into the electricalcircuit. To maintain an adequate lifespan of radio tag 29, the circuitryof radio tag 29 is preferably configured to operate substantially onleakage current of local power source 40. By operating radio tag 29 onleakage current only, radio tag 29 may obtain a long shelf life ofapproximately 10 years before replacement of the local power source 40is necessary.

The first and second frequency receivers 34 and 36 provide the dualfrequency capability of radio tag 29. In a particular embodiment, thefirst frequency receiver 34 is a VHF/UHF receiver operable to receivewakeup signals in the VHF/UHF frequency range, and the second frequencyreceiver 36 is a VLF/LF receiver operable to receive signals in theVLF/LF frequency range. In this embodiment, base station 11 transmitssignals in the VLF/LF frequency range and receives signals in theVHF/UHF frequency range, and radio tags 29 receive signals from basestation 11 in the VLF/LF frequency range, receive signals from otherradio tags 29 in the VHF/UHF frequency range, and transmit signals inthe VHF/UHF frequency range. As previously described, use by the basestation 11 of the VLF/LF frequency range allows base station 11 tocontrol the transmission of the signals. Use by the radio tags 29 of theVHF/UHF frequency range allows signals to be transmitted at low powerthus maximizing tag life.

The use of dual frequencies also allows the radio tag 29 to identify thetype of unit transmitting the signal. If radio tag 29 receives a signalin VLF/LF receiver 36, it knows that the message was transmitted by thebase station 11. Conversely, if radio tag 29 receives a signal onVHF/UHF receiver 34, it knows that the message was transmitted orforwarded by another radio tag 29.

In an exemplary embodiment, a low frequency of 132 kHz is used for theVLF/LF frequency range. The VHF/UHF frequencies are chosen based onlocal governmental restrictions on the use of these frequencies. In theUnited States, the VHF/UHF frequency may be a UHF frequency of 315 MHz.For European countries, the VHF/UHF frequency may be a UHF frequency of433 MHz.

Tag processor 30 receives a message, or communicated signal, anddetermines if a response is necessary. If tag processor 30 determinesthat a response should be sent to base station 11 or another radio tag,transmitter 32 modulates the response signal from tag processor 30 ontoan analog carrier wave and transmits the modulated signal via internalantenna 38.

Before tag processor 30 will process an incoming message, a wakeupsignal signature may be required. This allows the tag processor 30 toremain in a low power, or sleep, state during periods of inactivity whenthe radio tag is outside the range of the base station 11 or isotherwise not communicating. The wakeup signal signature informs tagprocessor 30 that information requiring processing will arriveimmediately following the wakeup signal signature.

Transmitter 32 modulates signals received from processor 30 onto acarrier wave for transmission via internal antenna 38. RFID system 10preferably utilizes amplitude modulation to carry the communicatedsignal on the carrier wave. It will be understood that other suitablemodulation schemes may be used. In the embodiment in which the tagscommunicate at a frequency within the VHF/UHF frequency range, thetransmitter 32 is a VHF/UHF transmitter. As previously described, theVHF/UHF frequency range is characterized by low noise in the frequencyspectrum. However, signals transmitted in this frequency range are notcontrollable. Thus, signals are dispersed in all directions. The lownoise characteristic of VHF/UHF frequencies allows low powered signalsto be transmitted. Since power consumption should be kept to a minimumso that radio tags in RFID system 10 obtain a maximum life expectancy, alow powered transmitter should be utilized. Although radio tags 29transmit high powered signals in the VHF/UHF frequency range and canobtain an operational range up to 1,000 feet, a particular embodimentlimits transmitted signal power such that the operational range of radiotags 29 is limited to approximately 150 feet.

FIG. 3 illustrates the second frequency receiver 36 of FIG. 2 inaccordance with one embodiment of the present invention. In thisembodiment, second frequency receiver 36 is a low power high sensitivityVLF/LF receiver. Although this embodiment utilizes the VLF/LF frequencyrange, other suitable frequency ranges may be used.

Referring to FIG. 3, second frequency receiver 36 includes a tunercircuit 42 that is tuned to the transmitting frequency of base station11 and a demodulator 44. Tuner circuit 42 is coupled to internal antennasystem 38 and receives a signal 48 therefrom.

Tuner circuit 42 eliminates extraneous signals which may cause falseactivations of radio tag 29 by filtering frequencies other than a targetfrequency. Tuner circuit 42 may include conventional tuner circuitrysuch as a high pass filter comprised of a capacitor in series with aresistor connected to a ground. Tuner circuit 42 effectively narrows thebandwidth of the signal 48 received and forwarded by internal antennasystem 38. In an exemplary embodiment, tuner circuit 42 is set to 132KHZ but may be set to any suitable frequency.

After tuner circuit 42 obtains and filters the received signal 48, thetuned and filtered signal is forwarded to demodulator 44. Demodulator 44demodulates the received signal and extracts the communicated signal inthe form of a square wave, or digital bit stream, for processing byprocessor 30. As described in more detail below, demodulator 44 has ahigh sensitivity so that it can detect and demodulate low poweredsignals. In one embodiment, the demodulator has a sensitivity of lessthan 10 millivolts. As a result, the radio tag 29 can detect and processweak signals at LF/VLF which may significantly increase the operationalrange of radio tag 29 to 150 feet and beyond.

After demodulator 44 extracts the communicated signal from the carrierwave, the communicated signal is forwarded to tag processor 30 forprocessing. As previously described, before processor 30 will process anincoming message, a wakeup signal may be required. The wakeup signalsignature may also be used to eliminate false activations of radio tag29. False activations may occur when utilizing a low threshold voltagelevel in demodulator 44. False activations of radio tag 29 areeliminated in part by tuner circuit 42 that only allows signals at atarget frequency to pass into demodulator 44. By utilizing a wakeupsignal signature, the remainder of potential false activations may beeffectively eliminated or reduced while still providing a highsensitivity demodulator.

The demodulator 44 includes a comparator 52 and a threshold voltagegenerator 54. The threshold voltage generator 54 generates a thresholdvoltage signal that is used by the comparator 52 to demodulate anincoming signal and determine if a communicated signal is present.Comparator 52 should be in an active state at all times so that it isprepared to sample any signal that may be obtained by internal antenna38. Since comparator 52 is always active, threshold voltage generator 54is always active and both continuously draw current from local powersource 40. The comparator provides high sensitivity at low powerconsumption levels and this increases the range of the radio tag 29without limiting its effective life.

Comparator 52 has a first input 50, a second input 56, and an output 58.The first input 50 is coupled to the tuner circuit 42 after receivessignal 48 has been tuned and filtered to receive the tuned signal. Thesecond input 56 is coupled to the threshold voltage generator 54 toreceive the threshold voltage signal. The output 58 is coupled to tagprocessor 30.

The comparator 52 is connected to local power source 40 and preferablyoperates on leakage current of the local power source 40. In oneembodiment, the link between local power source 40 and comparator 52 hasa capacitor 68 for regulation of the electrical current supplied tocomparator 52. This prevents fluctuations in local power source 40current from adversely affecting operation of the comparator 52.

In operation, comparator 52 demodulates the tuned signal by comparing itto the threshold voltage signal generated by threshold voltage generator54. The digital output on output 58 is based on the results of thecomparison. If the tuned signal exceeds the threshold voltage signal,comparator 52 generates a high output for the digital signal on output58. If the tuned signal is less than threshold voltage signal 56,comparator 52 generates a low output on digital signal 58. A high outputcorresponds to an “on” bit, and a low output corresponds to an “off”bit. Tag processor 30 receives the digital signal on output 58 anddetermines an appropriate response to be transmitted to base station 11or other radio tags.

The propagation delay of comparator 52 determines the sampling rate ofthe incoming signal. The propagation delay of comparator 52 is dependenton the specific frequency which must be captured, demodulated, andprocessed. Generally, a propagation delay of seven to ten times lessthan the period of the carrier wave being processed is sufficient tofully capture the communicated signal. Thus, seven to ten samples wouldbe taken of the incoming signal during each period. In the exemplaryembodiment where the carrier wave has a frequency of 132 KHz, thecomparator 52 has a period of 7.6 microseconds and a propagation delayof 900 nanoseconds which yields approximately eight samples per cycle ofthe carrier wave. The tag processor 30 accumulates and interprets thesamples.

Conventional comparators satisfy the low power requirements but areimpractical for application to an RFID tag as the propagation delay atlow overdrive signals (i.e. 10 mv) is in excess of twelve microseconds.As described below, the comparator 52 may be implemented by fabricatingthe novel comparator.

In a preferred embodiment, comparator 52 may be implemented in anapplication specific integrated circuit (ASIC). In this embodiment, thecomparator 52 may have a conventional design but be fabricated usingcomplimentary metal oxide semiconductor (CMOS) techniques at a sub 0.35micron process, such as Hewlett-Packard's MOSES process. Comparator 52of this embodiment has a typical propagation delay of 900 nanosecondsand a maximum propagation delay of one microsecond at less than 10millivolts overdrive using two to three microamps of current from alocal power supply 40 (lithium coin cell battery) over an extendedtemperature range with CMOS compatible outputs on a 0.35 micron process.Comparator 52 is implemented using a low power, sub-micron CMOS processto properly balance the tradeoffs between power consumption andperformance that currently available comparators inadequately address.

An exemplary embodiment utilizes a comparator with a propagation delayof 900 nanoseconds and a power consumption between two and threemicroamps. Since threshold voltage generator 54 draws approximately 997nanoamps and comparator 52 draws approximately three microamps, thetotal power consumption of demodulator 44 is less than four microampswhich will yield an adequate shelf life and is less than the leakagecurrent of local power source 40.

The threshold voltage generator 54 may be implemented as a voltagedivider circuit. In this embodiment, the threshold voltage generator 54has a first resistor 60, a second resistor 62 and a capacitor 66. Afirst resistor 60 is coupled to local power source 40. First resistor 60should have a resistance high enough to minimize the leakage currentflow while still providing sufficient leakage current to generatethreshold voltage signal 56. If the resistance of first resistor 60 istoo high, insufficient current will be available to generate thresholdvoltage signal 56. If the resistance of first resistor 60 is too low,excessive current will be drawn from local power source 40 andinadequate shelf life will result.

A second resistor 62 is placed in series with first resistor 60 and isconnected to a ground 64. The resistance of second resistor 62 is chosento generate the appropriate voltage for threshold voltage signal 56. Byvarying the resistance of second resistor 62, the sensitivity ofdemodulator 44 may be varied. An increase in the resistance of secondresistor 62 results in lower sensitivity of demodulator 44.

Capacitor 66 is connected across second resistor 62 in order to insurethat a constant voltage is provided on threshold voltage signal 56.Capacitor 66 insures that the sensitivity of comparator 52 is constantand does not fluctuate with any fluctuations in the current supplied bylocal power source 40.

In the exemplary VLF/LF embodiment, a three megaohm resistor is used asfirst resistor 60 and a 6.8 kiloohm resistor is used as second resistor62. The total current drawn by threshold voltage generator 54 is I=E/R,where I is current in amps, E is energy in volts, and R is resistance inohms. Utilizing a three volt local power source 40 and having a totalresistance from the resistances of first resistor 60 and second resistor62 of 3,006,800 ohms, the total current drawn by the threshold voltagegenerator 54 is 3V/3,006,800 ohms=997 nanoamps. Therefore, thresholdvoltage generator 54 draws less than 1 microamp from battery 44. Thethreshold voltage signal may be calculated as E(R2)/(R1+R2), which is3V(6800 ohms)/(3,006,800 ohms), or 6.8 millivolts. Therefore, anyreceived signal which exceeds 6.8 millivolts may be processed.

FIG. 4 illustrates details of the first frequency receiver 34 inaccordance with one embodiment of the present invention. In thisembodiment, the first frequency receiver 34 is a low power superregenerative receiver. Other suitable receivers and receiver designs maybe used. Power control functions of a superregenerative receiver permitsome of the elements of VHF/UHF receiver 34 to be completely shut downto save tag power while operating in a quiescent state. Thesuperregenerative receiver 34 includes the amplification stage 101, thelocal oscillator 102 operating at the certain frequency, quenchfrequency source 103 operating at a duty cycle of at least 10 times thedata rate, and a detector stage 104. The design utilizes forward biasingas provided by the quench frequency such that power draw is limited to50% of the normal 100% biasing techniques due to the 50% duty cycle ofthe quench. Amplifier power 101, is limited to less than leakagecurrents only (generally less than 500 nanoamps) when the quenchfrequency is shut down.

Additionally, the local oscillator (LO) 102 is controlled by the quenchfrequency such that it is turned off prior to achieving stableoscillation. In this way, the RF is sampled as the LO 102 is able toachieve stable oscillation significantly faster in the presence of an RFsignal than without an RF signal. The detector circuit simply filtersout the quench and LO frequencies (low pass filters) leaving the pulsecreated by the increased size of the RF envelope with RF present.

Since the LO 102 is also turned on and off by the quench frequency,power can also be controlled in the same way as the front end amplifier101 biasing described above. A Surface Acoustic Wave (SAW) Delay Line108 (in a preferred embodiment Model No. SL1011 from RF Monolithics, andin alternate embodiments any of the SLXXXX series of devices orequivalents) provides stability to the LO frequency and inserts theproper timing for signal reception 100, amplification 101, and quench103 sampling of the LO 102.

In the configuration illustrated in FIG. 4, the quench frequencyoscillator 103 actually includes two separate oscillators, first andsecond stage quench oscillators 105 and 106, that operate in one of twomodes and that are referred to as quench oscillator full power mode andquench oscillator low power mode. In low power quiescent mode, theoscillator 103 outputs a quench signal 120 as in FIG. 5. A duty cycle of1 to 5% over a period of 10 ms provides sufficient time for reception ofa 20 to 30 ms activation signal and reduces total circuit draw by asmuch as 99%. The high pulse consists of a 30 to 500 khz 50% duty cycletrapezoidal pulse train for normal quenching of the local oscillator102. When a signal is detected, microcontroller 107 turns off the lowduty cycle such that the 50% 30 to 500 khz normal quench frequencysignal 130 is maintained for normal data retrieval.

The detector circuit 104 is a micropower diode/comparator arrangement,although other more efficient types of detectors can be implemented inalternate embodiments as long as the power requirement is 1 microamp orless to minimize the total circuit power requirement. In the detectorcircuit 104, low power consumption is achieved through use ofconventional low power componentry, e.g., in a preferred embodiment amodel MAX 417 dual op-amp device from Maxim (not shown). In a preferredembodiment, detector circuit 104 operates as follows: The output ofquench frequency oscillator 103, as integrated with the local oscillator102, is first passed through a low pass filter (not shown) and appliedto the first op-amp of the Maxim device to amplify the resulting signalsufficiently to be applied to the second op-amp of the Maxim device,which is configured as a comparator creating a data pulse from detectorcircuit 104 when triggered. This data pulse is then applied tomicrocontroller 107 to indicate that quench frequency oscillator 103should be placed in full power mode by setting the second stage quenchoscillator output to a stable high output state 140.

The quench frequency oscillator 103 comprises the first stage quenchoscillator 105 and the separate, second stage quench oscillator 106. Thefirst stage quench oscillator 105 is coupled to the local oscillator 102and operable, when active, to activate the local oscillator 102. Thesecond stage quench oscillator 106 is coupled to the first stage quenchoscillator 105 and operable in a low power mode to periodically activatethe first stage quench oscillator 105 in order to periodically activatethe local oscillator 102 for the purpose of detecting the presence of acommunicated signal and in a full power mode to continuously activatethe first stage quench oscillator in order to continuously activate thelocal oscillator 102 for the purpose of collecting the communicatedsignal. When continuously activated, the local oscillator 102 and firststage quench oscillator 105 may each have a fifty percent or othersuitable duty cycle for full or desired sensitivity reception. Thecommunicated signal is a signal communicated to the radio tag 29. Thecommunicated signal may be an ultra high frequency (UHF) or othersuitable signal. It will be understood that the quench oscillator 102may instead be a dual quench oscillator and that the receiver maycomprise other types of suitable receivers operating on a limited powersupply, such as a coin cell battery, and may include other suitablecircuits and components.

FIG. 6 is a flow diagram illustrating operation of the superregenerativereceiver 34 in accordance with one embodiment of the present invention.Referring to FIG. 6, the method begins at state 113 in which the quenchoscillator 103, and thus the receiver 34, is in the low power mode. Theradio tag 29 is in sleep or stand-by mode. In the low power mode, thereceiver 34 preferably uses only leakage current from a battery for theradio tag 29. For a radio tag 29 operating on a typical lithium coincell battery, for example, the receiver 34 in the low power mode uses500 nanoamps or less power. As a result, the receiver 34 need not useactive current from the battery, and life of the battery and the radiotag 29 are extended.

In the low power mode, the second stage quench oscillator 106 has a lowduty cycle that periodically generates a first stage activation, orsampling, signal to detect whether a signal is present. The duty cyclemay be less than five percent and in the preferred embodiment is aboutone percent. The duty cycle should be sufficient to enable sampling oftraffic being received such that the presence of signals can be detectedwhile minimizing power consumption.

In response to the periodic sampling signal, state 113 transitions tostep 114 in which the first stage quench oscillator 105 is activated bythe sampling signal. In a particular embodiment, the first stage quenchoscillator 105 is active only in the presence of the sampling signal.Thus, the first stage quench oscillator 105 will activate based on theduty cycle of the second stage quench oscillator 106.

Proceeding to step 115, the first stage quench oscillator 105 generatesa local oscillator activation signal. At step 116, the local oscillator102 is activated in response to the activation signal from the firststage quench oscillator 105. In a particular embodiment, the localoscillator 102 is acting only in the presence of the local oscillatoractivation signal. Thus, in the low power mode, the local oscillator 102will activate based on the duty cycle of the second stage quenchoscillator 106.

Next, at step 117, the local oscillator 102 demodulates received trafficto generate a demodulated signal. The local oscillator 102 demodulatesreceived traffic at a specified frequency. The communicated signals aretraffic modulated at that specified frequency.

Proceeding to decisional step 118, the detector 104, in combination withthe microcontroller 107, determines whether a communicated signal ispresent in the demodulated signal output by the local oscillator 102. Ifa communicated signal is not present, the radio tag 29 may return tostand-by, or sleep mode. Accordingly, the NO branch of decisional step118 returns to the low power mode at state 113 in which the second stagequench oscillator 106 remains at the low duty cycle to minimize powerconsumption.

Returning to decisional step 118, if a communicated signal is present inthe demodulated signal, the communicated signal needs to be collectedand the YES branch of decisional step 118 leads to state 119. At state119, the second stage quench oscillator 106, and thus the quenchoscillator 103 and the receiver 34, transition to full power mode. Inthe full power mode, the second stage quench oscillator 106 has a fullduty cycle to continuously activate the first stage quench oscillator105. In response, the first stage quench oscillator 105 continuouslyactivates the local oscillator 102 for full sensitivity reception andthe communicated signal is demodulated and collected. Accordingly, fullpower is used only when a communicated signal is present and needs to becollected.

The receiver 34 remains at state 119 until the communicated signal hasbeen fully received. After complete reception of the communicatedsignal, in response to a timeout or other suitable event, state 119returns to decisional step 119 in which it is determined if anothercommunicated signal is present. If a communicated signal is present andbeing received, the receiver 34 is returned to state 119 and remains infull power mode at least until the communicated signal is fullycollected. Following collection of the communicated signal, and theabsence of a further communicated signal, the NO branch of decisionalstep 118 returns to state 113 in which the second stage quenchoscillator 106, and thus the receiver 34, are in the low power mode.Accordingly, the receiver 34 is maintained in full power mode only aslong as necessary to collect a communicated signal and, if desired, fora short period thereafter. In this way, by using a second mode ofoperation or a second quench oscillator, at a substantially lowerfrequency, substantial power savings are realized. In the low powermode, sampling the radio frequency takes place at a duty cycle that isconducive to long battery life. Once a radio frequency input signal isdetected, the higher frequency quench is turned on and full sensitivityis achieved. This could all be timed such that the full turn on of theunit is accomplished during transmission of a preamble from thetransmitting device.

The microcontroller 107 provides binary outputs to control the mode ofoperation. The controller 107 operates in a low power sleep state untilthe pulse from the low power, low duty cycle quench is detected via someincoming radio frequency and is awakened. The controller 107 thenimmediately upon awakening turns off the low duty cycle mode and turnson the normal quench frequency controller and searches for valid radiofrequency pulses for demodulation. Once the pulses have stopped for someperiod, the controller 107 turns off normal quench, turns on low powerquench and goes back to low power sleep mode.

FIG. 7 illustrates an asset tracking system 129 in accordance with oneembodiment of the present invention. In this embodiment, an automobilestorage facility for temporarily storing vehicles before the vehiclesare transported to retail facilities is used to describe the operationof the asset tracking system. In that embodiment, vehicles may be storedat the automobile storage facility after manufacture or afterimportation. The asset tracking system 129 of the present invention maybe used with other suitable applications where inventory or other itemsof interest are distributed over large areas.

Referring to FIG. 7, the asset tracking system 129 includes a basestation 120 and a tag network 125. The base station 120 is similar tothe previously described base station 11. Tag network 125 includes aplurality of dual frequency radio tags 130-165. Dual frequency radiotags 130-165 are similar to the previously described radio tag 29 andare operable to transmit and/or receive on multiple frequencies. It willbe understood that single frequency tags may be used within the tagnetwork 125.

The base station 120 may include a dual mode antenna as previouslydescribed or separate antennas for the different frequency rangestransmitted and received by base station 120. For a VHF/UHF and VLF/LFembodiment, base station 120 uses separate antennas for VHF/UHFfrequencies and VLF/LF frequencies. A VHF/UHF antenna 121 is coupled tobase station 120 and is operable to receive messages from dual frequencyradio tags 130-165 transmitted on the VHF/UHF frequency. The VLF/LFfrequency range is used to transmit signals from base station 120 toradio tags 130-165. Because VLF/LF frequencies have limited transmissiondistance, a large loop antenna is used to disperse the messagetransmitted on the VLF/LF frequency over an area determined to reach anumber of radio tags within radio tags 130-165 in order to propagate themessage over the entire vehicle storage facility.

In a particular embodiment, VLF/LF antenna 122 is a 20 foot by 100 footloop antenna. Base station 120 generates signals at a power leveldetermined to disperse the signals approximately 15 to 20 feet on eitherside of VLF/LF antenna 122. Thus, base station 120 has a VLF/LFfrequency operational range 126 of approximately 60 feet by 140 feet.The operational range is the maximum area covered by a device or asuitable portion of that area. Radio tags within the operational rangeof base station 120 will receive signals from base station 120 on aVLF/LF frequency receiver. Radio tags outside the operational range ofbase station 120 will receive signals propagated through the tag network125 using the previously described retransmission capability of theradio tags 130-165. Each radio tag 130-165 in tag network 125 has anoperational range 127 illustrated by hashed circles.

A specific example of an asset tracking system in accordance with theasset tracking system illustrated in FIG. 7 is a vehicle storagefacility used for storing imported vehicles after the vehicles areunloaded from a ship. Before entering the vehicle storage facility, adual frequency radio tag similar to radio tag 29 is attached orotherwise coupled to each vehicle for tracking and identificationpurposes. An asset tracking system used in this way will allow efficienttracking of a large number of vehicles held in inventory while awaitingtransport to retail facilities across the country. The tag network 125of FIG. 7 will be discussed referring specifically to the import vehiclelot example.

In the vehicle storage facility embodiment, outbound messages from basestation 120 are generally non-explicit communications such as aninventory request message, and inbound messages are generally explicitcommunications, such as responses to base station 120 for an inventoryrequest message.

Base station 120 may be located anywhere in the vehicle storagefacility. The operational range of base station 120 and radio tagswithin tag network 125 are sized such that at least one radio tag iswithin the operational range of base station 120 and all other radiotags in tag network 125 are within the operational range of at least oneother radio tag. Sizing the operational ranges of base station 120 andradio tags in tag network 125 ensures that messages sent by base station120 or a radio tag in tag network 125 reach an appropriate destinationor destinations.

In operation, base station 120 may periodically take inventory of thevehicle storage facility, verify that a particular vehicle is within thefacility or obtain locational or other information about a vehicle. Basestation 120 may be loaded with expected inventory by entering eachvehicle into inventory as it enters the vehicle storage facility andremoving each vehicle from inventory as it exits the vehicle storagefacility. After base station 120 takes inventory of the vehicle storagefacility, base station 120 may compare the inventory results with theexpected inventory in order to identify potentially missing vehicles.

When base station 120 takes a periodic inventory of the vehicle storagefacility, base station 120 generates a non-explicit message requestingthat all radio tags 130-165 in tag network 125 respond. Recall thatnon-explicit messages are broadcast messages with no specific target ordestination address. Thus, all radio tags 130-165 in tag network 125receiving the message will respond to base station 120 and forward themessage to other radio tags. The responses received by base station 120can be compared to the expected inventory and missing, or unaccountedfor, vehicles can be identified.

Communications within an asset tracking system 129 utilizeacknowledgments and resending of messages in order to ensure thatmessages are received as intended. After a message is transmitted, anytag receiving that message will issue an acknowledge response or anunacknowledged response. An acknowledge response informs thetransmitting radio tag that the message was successfully received. Anunacknowledged response informs the transmitting radio tag that only aportion of the message was received and that the message should beretransmitted. If the transmitting device does not receive anacknowledge or unacknowledged response within a specified period oftime, the transmitting device will retransmit the message a fixed numberof times to insure that any radio tag within the operational range ofthe transmitting device receives the message. In a particularembodiment, messages are retransmitted a maximum of three times. If theacknowledge message is not received within a predetermined time, themessage is retransmitted a fixed number of times before the destinationradio tag is considered not found or missing. Since a message can beretransmitted and received multiple times by a radio tag, radio tagsinclude logic to prevent a particular radio tag from responding to aparticular message more than one time. In one embodiment, the logic toprevent multiple responses to a particular message includes adding atransaction number to each message so that a radio tag only respondsonce to a particular transaction number. In one embodiment, thetransaction number is randomly generated at the radio tags andsequentially generated at the base station.

In the exemplary embodiment, base station 120 transmits the non-explicitinventory request message on the VLF/LF frequency via VLF/LF antenna122. Radio tags in tag network 125 that exist within the operationalrange 126 of base station 120 receive the inventory request message onan internal receiver similar to the previously described secondfrequency receiver 36. The receiving radio tags 130-132 acknowledgereceipt of the message, process the message and respond accordingly, andretransmit the message to other radio tags within tag network 125. Theinventory request message is retransmitted through tag network 125 suchthat all radio tags receive the message. As responses are generated byradio tags in tag network 125, other radio tags receive and retransmitthe responses until they reach base station 120.

In the exemplary embodiment, radio tags 130-132 exist within theoperational range 126 of base station 120 and receive the inventoryrequest message on a VLF/LF receiver 36 in each radio tag. The inventoryrequest message is preceded by a wake up signal identifier. Radio tag130 receives the inventory request message and generates an explicitcommunication to base station 120 acknowledging receipt of the inventoryrequest message. Radio tag 130 then generates an appropriate response tobase station 120. A response generated by a radio tag will generallyidentify the address of the radio tag generating the response and thedestination address of the device to receive the response. In this case,the response generated by radio tag 130 would identify radio tag 130 asthe generating device address and base station 120 as the destinationdevice address since base station 120 initiated the inventory requestmessage.

After transmitting a response to base station 120, radio tag 130retransmits the inventory request message through its internal VHF/UHFtransmitter 32. Radio tag 140 receives the inventory request message onits internal VHF/UHF receiver 34. Radio tag 140 first acknowledgesreceipt of the inventory request message and then generates andtransmits an appropriate response to the inventory request message.Radio tag 140 then retransmits the inventory request message to otherradio tags. Radio tag 130 receives the acknowledge message and theresponse generated by radio tag 140. Since the response is an explicitcommunication directed toward base station 120, radio tag 130retransmits the response to base station 120.

The inventory request message continues in tag network 125 through radiotags 141-145. Each radio tag 141-145 receives the inventory requestmessage, generates an acknowledge message to the radio tag thatforwarded the inventory request message, generates an appropriateresponse, and retransmits the original inventory request message. Eachradio tag in the tag network 125 may receive and retransmit an explicitresponse directed to base station 120. In this way, inventory requestmessages generated by base station 120 are propagated through tagnetwork 125 and any responses directed to base station 120 flow throughthe tag network 125 to base station 120.

Radio tag 150 is outside the operational range 126 of both base station120 and, the operational range 127 of radio tag 130. However, it iswithin the operational range 127 of radio tag 140. Therefore, when radiotag 140 retransmits the inventory request message, radio tag 150receives the inventory request message, generates an acknowledge messageto radio tag 140, and generates an appropriate response to base station120. Radio tag 150 then propagates the inventory request message throughradio tags 150-152 in tag network 125 by retransmitting the inventoryrequest message.

Radio tag 132 is within the operational range 126 of base station 120and functions as the link between base station 120 and radio tags160-165 in tag network 125. Messages are transmitted and receivedthrough radio tags 160-165, in tag network 125 as previously discussedwith relation to radio tags 140-145 and radio tags 150-152.

Radio tag 131 exists within the operational range 126 of base station120 and receives and responds accordingly to the inventory requestmessage generated by base station 120. Because radio tags already withinthe operational range 127 of radio tag 131 are activated by other tags,radio tag 131 does not further activate any other radio tags in tagnetwork 125 and does not perform communications between radio tags intag network 125.

FIG. 8 illustrates the operation of a radio tag in tag network 125 inone embodiment of asset tracking system 129. Before the processillustrated in FIG. 8 commences, the radio tag receives an appropriatewakeup signature and prepares to receive an incoming message. Theincoming message could be a non-explicit communication from base station120, an explicit communication from base station 120, a messageretransmitted by another radio tag, an acknowledge message, anunacknowledged message or other suitable message.

The method begins at step 170 where a dual frequency radio tag receivesa signal containing a message. Internal antenna system 38 obtains thesignals and forwards the signals to VHF/UHF receiver 34 and VLF/LFreceiver 36. VHF/UHF receiver 34 and VLF/LF receiver 36 tune anddemodulate the signal to generate a digital bit stream containing themessage for processing by processor 30.

The method proceeds to decisional step 171 where processor 30 determineswhether a complete message has been received. If a complete message hasnot been received, the NO branch of decisional step 171 proceeds to step172 where the radio tag generates and transmits an unacknowledgedmessage. After step 172, the process terminates and waits forretransmission of the partially received message.

If a complete message has been received either initially or uponretransmission, the YES branch of decisional step 171 proceeds todecisional step 173 where processor 30 determines if the message is anacknowledge message for a message recently transmitted by the radio tag.If the message is an acknowledge message for a message recentlytransmitted by the radio tag, no further processing is necessary and theYES branch of decisional step 173 leads to the end of the method. If themessage is not an acknowledge message, the NO branch of decisional step173 leads to decisional step 174 where processor 30 determines whetherthe message is an unacknowledged message for a recently transmittedmessage by the current radio tag. If the message is not anunacknowledged message for a recently transmitted message by the currentradio tag, the NO branch of decisional step 174 proceeds to step 176where the radio tag generates and transmits an acknowledge message toacknowledge receipt of the current message.

Proceeding to decisional step 178, the tag processor 30 determineswhether the message is an explicit communication. If the message is anexplicit communication, the YES branch of decisional step 178 proceedsto decisional step 180 where the processor 30 determines whether theradio tag is the destination for the explicit message. If the currentradio tag is the destination for the explicit message, the YES branch ofdecisional step 180 proceeds to step 182 where the radio tag generatesand transmits an explicit response to the originating device, in thisembodiment, base station 120.

The method proceeds to decisional step 191 where the radio tagdetermines if an acknowledge message has been received within the timeout period. As previously discussed, the time out period is a waitingperiod before the radio tag will retransmit the message. If anacknowledge message is received within the time out period, the YESbranch of decisional step 191 leads to the end of the method. If anacknowledge message is not received within the time out period or uponreceipt of an unacknowledged message, the NO branch of decisional step191 proceeds to decisional step 192 where the radio tag determineswhether the message has been retransmitted the maximum number of times.If the radio tag determines that the maximum number of retransmissionshas been reached, the YES branch of decisional step 192 leads to the endof the method. If the radio tag determines that the maximum number ofretransmissions has not been reached, the NO branch of decision step 192returns to step 182 where the response is transmitted.

Returning to decisional step 178, if the message is not an explicitcommunication, the NO branch of decisional step 178 proceeds to step 184where the radio tag generates and transmits an appropriate response tothe originating device. In the exemplary embodiment, base station 120 isthe originating device for non-explicit messages within the assettracking system 129. The method proceeds to decisional step 188 wherethe radio tag determines if an acknowledge message has been receivedwithin the time out period. As previously discussed, the time out periodis a waiting period before the radio tag will retransmit the message. Ifan acknowledge message is received within the time out period, the YESbranch of decisional step 188 leads to the end of the method. If anacknowledge message is not received within the time out period or uponreceipt of an unacknowledged message, the NO branch of decisional step188 proceeds to decisional step 190 where the radio tag determineswhether the message has been retransmitted the maximum number of times.If the radio tag determines that the maximum number of retransmissionshas been reached, the YES branch of decisional step 192 leads to the endof the method. If the radio tag determines that the maximum number ofretransmissions has not been reached, the NO branch of decision step 192returns to step 184 where the response is transmitted.

Returning to decisional step 174, if the received message is anunacknowledged message, the YES branch of decisional step 174 proceedsto step 186 where the message that was partially received by anotherdevice is retransmitted. Similarly, at decisional step 180, if thecurrent radio tag is not the destination for the explicit message, theNO branch of decisional step 180 proceeds to step 186 where the currentmessage is retransmitted.

After step 186, the method proceeds to decisional step 193 where theradio tag determines if an acknowledge or unacknowledged message hasbeen received within the time out period. As previously discussed, thetime out period is a waiting period before the radio tag will retransmitthe message. If an acknowledge message is received within the time outperiod, the YES branch of decisional step 193 leads to the end of themethod. If an acknowledge message is not received within the time outperiod or an unacknowledged message is received, the NO branch ofdecisional step 193 proceeds to decisional step 194 where the radio tagdetermines whether the message has been retransmitted the maximum numberof times. If the radio tag determines that the maximum number ofretransmissions has been reached, the YES branch of decisional step 194leads to the end of the method. If the radio tag determines that themaximum number of retransmissions has not been reached, the NO branch ofdecision step 194 returns to step 186 where the appropriate message isretransmitted.

FIG. 9 illustrates a distributed asset control system 200 in accordancewith another aspect of the present invention. In this embodiment, thedistributed asset control system 200 may be used, for example, to trackexpensive garments on hanging racks located throughout a retailfacility. Distributed asset control system 200 may be used in a varietyof situations including other retail and office asset management systemsto monitor assets.

Referring to FIG. 9, the distributed asset control system 200 includes abase station 202 and a plurality of primary tags 210. Each primary tag210 is functionally associated to a set of secondary tags 212. A dualmode antenna 204 as previously described is part of base station 202. Inan exemplary embodiment, primary tag 210 is a dual frequency radio tag29 as previously described. The base station 202 includes a reader andcontrol system similar to base station 11. The base station 202 islocated in a facility to be monitored such that at least one primary tag210 is within the operational range 205 of base station 202. Theoperational range 211 of each primary tag 210 is set such that a messagetransmitted by base station 202 will be received and propagated to eachprimary tag 210 within distributed asset control system 200.

Dual mode antenna 204 may transmit and receive signals in the VLF/LFfrequency range and the VHF/UHF frequency range. Although the presentembodiment will be discussed using three hanging racks 206, 207, and208, more hanging racks could be added to the distributed asset controlsystem 200 without requiring additional base stations 202. For theexemplary embodiment, a primary tag 210 is mounted on each hanging rack206, 207, and 208 and controls a set of associated secondary tags 212.Each secondary tag 212 is mounted on an article of clothing. Secondarytags 212 are preferably passive radio tags in order to reduce costs.However, secondary tags 212 may comprise active radio tags.

Each primary tag 210 within the distributed asset control system 200 isloaded with the inventory for the hanging rack to which the primary tag210 is attached. Periodically, base station 202 transmits updates to theprimary tags 210 accounting for garments sold or garments added to thehanging racks. Since a secondary tag 212 is attached to each garment,garments sold or added to hanging racks translates into secondary tagidentifications being removed or added to the expected inventory storedin primary tag 210. Each primary tag 210 is programmed to periodicallyinitiate an inventory control function. Thus, the inventory control ofeach hanging rack 206, 207, and 208 is performed by the primary tag 210attached to the hanging rack. By distributing the inventory controlfunction into primary tags 210, more available bandwidth and computingpower of base station 214 is available for monitoring store exits,sales, new inventory, and other functions.

Each primary tag 210 periodically transmits a non-explicit inventoryrequest message. Each secondary tag 212 within the set of secondary tags212 associated with each primary tag 210 responds identifying itself asa secondary tag 212 within the operational range 211 of a primary tag210. Each primary tag 210 receives the responses and generates amonitored asset status by comparing the responding secondary tag 212identification information to the expected inventory. If there arediscrepancies, such as missing or additional garments, an alarm messageidentifying the missing or extra garments is generated and transmittedto base station 202. Each primary tag 210 is operable to retransmitexplicit messages destined for a different device address as previouslydescribed. Therefore, an inventory report or alarm message generated bya primary tag 210 may be retransmitted to base station 202 by anotherprimary tag 210. Similarly, when base station 202 transmits messages toprimary tags 210, the primary tags 210 are operable to retransmit themessage so that all primary tags 210 within distributed asset controlsystem 200 receive the message.

In addition to the periodic inventory control functions of primary tags210, base station 202 may periodically initiate its own inventorycontrol process, for example when the store opens and closes. Basestation 202 would transmit an inventory request message to all primarytags 210 within its operational range 205. Since the inventory requestmessage is a non-explicit communication, primary tags 210 retransmit themessage to other primary tags 210 and perform the requested inventorycontrol function. The results of the inventory control functions aretransmitted to base station 202 through the primary tag 210 chainutilizing tag to tag communications as previously described.

In another embodiment, the primary tags may be used in connection with acomputer, telephone, or other suitable device connected to a basestation over a wire line network. In that embodiment, the base stationgenerates a polling event, and the primary tag receives the pollingevent over the wireline connection and processes the polling event aspreviously described.

The distributed asset control system utilizing primary tags 210 andsecondary tags 212 may be used in various situations. Another example ofa situation where this type of system would be appropriate is assistingparents with tracking their children in a crowded environment such as anamusement park. A parent would wear a primary tag 210 and the childrenwould wear secondary tags 212. In this example, the secondary tags 212are preferably active radio tags in order to eliminate potentialenvironmental influences which may alter the operational range ofpassive radio tags and, therefore, provide inconsistent results. Usingactive radio tags provides for a consistent operational range betweenthe primary tag 210 and the secondary tag 212.

In the parent/child system, the primary tag 210 would be set toperiodically issue a polling request for its assigned secondary tags212. The time period is preferably a short time period such as 5-10seconds. The operational range of the primary tag 210 is set to arelatively short distance such as ten feet. Thus, as long as thesecondary tag 212 on each child remains within a ten foot radius of theprimary tag 210 on the parent, no error or alarm conditions result. If achild wearing the secondary tag 212 is not within the ten foot radius ofthe primary tag 210 when the primary tag 210 issues a poll, an alarmwould sound on the primary tag 210. The alarm would alert the parentthat a child is outside the specified range and investigative actioncould be started. The secondary tags 212 could also initiate periodicpolling to ensure that the primary tag 210 was within range. The systemcould also include a central monitoring station so that a missing childalarm could be immediately reported to a central facility so that exitscould be appropriately monitored.

In another embodiment, the distributed asset control system may be usedin a proximity detection application. For example, large hauler trucksor other large equipment that cannot readily see other vehicles in theirpath may be equipped with a primary tag that periodically pollssecondary tags in the other vehicles. Upon receiving a response from asecondary tag that indicates that the secondary tag is within theoperational range of the primary tag on the large hauler truck, theprimary tag generates an alarm indicating the proximity of othervehicles.

FIG. 10 illustrates the operation of a primary tag 210 in thedistributed asset control system 200 in accordance with one embodimentof the present invention. The method begins at step 220 where a primarytag 210 is initialized with the identification numbers of each secondarytag 212 that is functionally associated to primary tag 210. At step 222,the primary tag polls secondary tags 212 to determine or verify theexistence of associated secondary tags 212 within the operational rangeof the primary tag 210. Next, at step 224, secondary tags 212 receivingthe poll transmit a response. The responses include the address of thetag generating the response.

Proceeding to step 226, primary tag 210 compares the identifications inthe responses from secondary tags 212 to the linked list of secondarytags. The linked list includes the identification of the associatedsecondary tags. At decisional step 228, the primary tag 210 determines amonitored asset status based on whether the identifications in theresponses exist in the linked secondary tag list. If one or moresecondary tags on the linked secondary tag lists did not respond or oneor more secondary tags responded, the YES branch of decisional step 228proceeds to step 230 where an alarm condition with the monitored assetstatus of the most recent polling event is transmitted to the basestation 202. If there are no discrepancies between the linked list ofsecondary tags and the responding secondary tags, the NO branch ofdecisional step 228 proceeds to step 232 where an all normal messagewith the results of the most recent polling event is transmitted, whennecessary, to the base station 202. The results of the polling event aretransmitted only if the RFID system is designed such that base station202 is informed of each inventory conducted by the primary tags 210.

FIG. 11 illustrates an access control system 240 using primary tags andsecondary tags. Access control system 240 may be used, for example, in asecure facility 241. The access control system 240 may be used to trackhigh priced assets within secure facility 241 and to preventunauthorized removal therefrom.

Secure facility 241 may have several access doors 242. Each access door242 has an access controller 244 which may be networked to a centralcontrol computer (not expressly shown). A primary tag 248 is attached toan asset 246 such as a laptop computer or a personal computer. Primarytag 248 is preferably a dual frequency radio tag such as dual frequencyradio tag 29 previously described. Primary tag 248 has been previouslyloaded with the identification numbers of linked secondary tags 252which may possess and remove asset 246 from secure facility 241. Aperson 250 has a secondary tag 252. Active or passive secondary tag 252may be an identification tag. In one embodiment, only persons 250 with asecondary tag 252 that is linked to primary tag 248 on asset 246 arepermitted to remove asset 246 from secure facility 241.

In another embodiment, access control system 240 may be used to preventassets 246 from leaving secure facility 241. In that embodiment, primarytag 248 would have no authorized secondary tags 252 loaded in itsmemory. One skilled in the art will understand that the access controlsystem 240 can provide many levels of control. For instance, twoseparate assets which must be used together could be linked so that oneasset could not be moved without the other. Another example would bemandating that certain persons always leave the secure facility with acertain asset such as a pager.

In another embodiment, access control system 240 may be used as awarranty control and fraud detection system. In this embodiment,products with expensive parts may be monitored to determine if theoriginal parts are present when the product is submitted for warrantyrepair. For example, a computer has several expensive component partsthat may be interchanged thus subjecting the warranty repair facility tofraudulent requests for repairs when non-functioning, non-original partsare placed in the computer. In the exemplary embodiment, a computer hasa primary tag mounted inside the computer case. A secondary tag ismounted on each component part of interest. For instance, the motherboard, the processor chip, the power supply, the disk drive, the CD-ROMdrive, and any other expensive parts may have a secondary tag. Beforeleaving the manufacturing facility, the primary tag is loaded with afunctionally linked list of secondary tags.

When the computer is submitted for warranty work, a base station issuesa wakeup signal causing the primary tag to poll the surroundingsecondary tags to determine if all the original parts are present. Ifeach secondary tag on the linked list of secondary tags is present, theprimary tag issues an equipment list verification message to the basestation. If any original parts are missing, the primary tag would issuea missing parts message to the base station. If any parts are changedduring warranty repair, the primary tag is updated with the new list offunctionally linked secondary tags.

FIG. 12 illustrates the basic method of the access control system 240 ofthe present invention. The method begins at step 260 where primary tag248 is loaded with a list of linked secondary tags 252. The linkageidentifies which secondary tags 252 can or must accompany primary tag248 as it proceeds through an access door 242. Access door 242 may be anexit from secure facility 241 or an access between secured locationswithin secure facility 241.

During operation, at step 262, an access controller 244 issues a wakeupsignal. As person 250 approaches access door 242 with asset 246, primarytag 248 receives the wakeup and/or other suitable signal and messageindicative of an attempt being made to remove the asset of the tag fromthe secure facility 241. The method then proceeds to step 264 whereprimary tag 248 issues a polling request for tags within its operationalrange. At step 266, primary tag 248 receives, accumulates and processesthe responses from tags within the operational range of primary tag 248.

In processing responses, the primary tag determines if passage throughaccess door 242 is permitted. If a responding secondary tag is on thelist of linked secondary tags, exit may be approved. If the accumulatedresponses do not include a functionally linked secondary tag 252, exitthrough access door 242 may be denied. Next, primary tag 248 approvespassage through access door 242, the YES branch of decisional step 268proceeds to step 270 where primary tag 248 transmits an open doorrequest or other suitable signal indicating that passage should beallowed to access controller 244. At step 272, access controller 244receives the open door request and opens access door 242. If primary tag248 does not approve passage through access door 242, the NO branch ofdecisional step 268 proceeds to step 274 where primary tag 248 generatesand transmits an alarm signal to access controller 244. At step 276,access controller 244 sounds an alarm indicating that an unauthorizedperson is attempting to remove an asset from secure facility 241 andprevents exit from secure facility 241.

FIG. 13 illustrates an asset management system 280 utilizing a hierarchyof primary tags and secondary tags to ensure that equipment, goods,personal, and other items are properly matched. An example of an assetmanagement system 280 in accordance with the present invention is asecure trucking facility 281 having access gates 282. A gate controlsystem 283 controls access gates 282 providing entrance to and exit fromthe secure trucking facility 281. Secure trucking facility 281 includesa warehouse and loading dock (not expressly shown).

Referring to FIG. 13, a tractor 296 has a tractor tag 286. Tractor tag286 is a secondary tag and may be an active secondary tag or passivesecondary tag as previously discussed. Trailer 292 has a trailer tag288. Trailer tag 288 is an intermediate tag that functions as both aprimary tag and a secondary tag. Trailer tag 288 is similar to radio tag29 as previously discussed. Trailer 288 contains a set of cargo 292wherein each cargo box 294 has a cargo tag 295. Cargo tag 295 may be asecondary tag as previously discussed. A driver (not expressly shown)has a driver tag 284. Driver tag 284 may be a primary tag such as radiotag 29 as previously discussed.

Asset management system 280 utilizes a hierarchy of primary tags todistribute the asset management functions of the system. The first levelof primary tag would be the driver tag 284 that communicates directlywith gate control system 283. Driver tag 284 then controls the secondlevel of primary tag, the intermediate trailer tag 288. Trailer tag 288manages its linked secondary tags, cargo tags 295. Driver tag 284manages its functionally linked secondary tags, tractor tag 286 andtrailer tag 288.

A driver is assigned a driver tag 284 that has a list of linkedsecondary tags that must be present before driver tag 284 informs gatecontrol system 283 to open access gates 282. In the exemplaryembodiment, driver tag 284 has the identification number of tractor tag286 and trailer tag 288 on its functionally linked secondary tag list.Trailer tag 288 is loaded with a list of linked cargo tags 295 thatshould be present on the trailer 292.

The trailer 292 is loaded with cargo 294 at a loading dock. The drivercouples his assigned tractor 296 to the appropriate trailer 292 andproceeds to access gates 282. As the tractor trailer rig approaches gatecontrol system 283, gate control system 283 transmits a wakeup signal.Driver tag 284 receives the wakeup signal and transmits a pollingrequest for linked secondary tags within its operational range. Tractortag 286 and trailer tag 288 receive the polling request and issue aresponse identifying themselves. Driver tag 284 verifies that tractortag 286 and trailer tag 288 identify the expected tractor and trailercombination loaded in driver tag 284. After trailer tag 288 receives thepolling request from driver tag 284, trailer tag 288 also acts as aprimary tag and issues a polling request to the set of cargo tags 295.Each cargo tag 295 issues a response identifying itself to trailer tag288. Trailer tag 288 accumulates the responses from cargo tags 295 andcompares identifications in the responses to its list of linkedsecondary tags. If trailer tag 288 determines that a linked secondarytag is missing or that a responding cargo tag 295 is not on the linkedsecondary tag list, trailer tag 288 transmits a message to driver tag284 indicating that the cargo 290 loaded on trailer 292 is incorrect. Iftrailer tag 288 determines that the appropriate cargo 290 has beenloaded on trailer 292 based on matching the responding cargo tags 295with the linked list of secondary tags, trailer tag 288 transmits amessage to driver tag 284 indicating that the appropriate cargo 290 hasbeen loaded on trailer 292.

Driver tag 284 determines if the proper tractor 296 and trailer 292combination exists and whether trailer tag 288 has determined if trailer292 has been loaded with the correct cargo 290. Based on thisinformation, driver tag 284 transmits a message to gate control system283 either authorizing exit from secure trucking facility 281 orindicating that an error condition exists and must be investigated. Byutilizing the system, a secure trucking facility 281 may ensure that thecorrect cargo 290 is being transported to the correct location beforethe tractor-trailer rig leaves the secure trucking facility 281. Gatecontrol system 283 can be likewise configured to perform some or all ofdriver, tractor, trailer and cargo inquiries. However, by distributingcertain processing into the radio tags, bandwidth savings are realizedand gate control system 282 has increased processing capabilities.

It is apparent that there has been provided in accordance with thepresent invention a system for communications between radio tags whichsatisfies the advantages set forth above such as providingcommunications between radio tags and distributed processing capabilityfor the RFID system. Although the present invention and its advantageshave been described in detail, it should be understood that variouschanges, substitutions, and alterations readily apparent to thoseskilled in the art may be made without departing from the spirit and thescope of the present invention as defined by the following claims.

1. A radio frequency tag, comprising: an antenna system; a firstreceiver coupled to the antenna system and operable to receive andprocess request signals from a first unit type at a first frequency; asecond receiver coupled to the antenna system and operable to receiveand process request signals from a second unit type at a disparatesecond frequency; and a transmitter coupled to the antenna system andoperable to transmit response signals at the second frequency.
 2. Theradio tag of claim 1, wherein the second unit type has a limited powersupply and the first frequency is a lower frequency than the seconddisparate frequency.
 3. The radio tag of claim 2, wherein the firstfrequency is a low frequency (LF).
 4. The radio tag of claim 2, whereinthe first frequency is a very low frequency (VLF).
 5. The radio tag ofclaim 2, wherein the first frequency is a microwave frequency.
 6. Theradio tag of claim 2, wherein the second frequency is a very highfrequency (VHF).
 7. The radio tag of claim 2, wherein the secondfrequency is an ultra high frequency (UHF).
 8. The radio tag of claim 2,wherein the first unit type is a base station and the second unit typeis another radio frequency tag.
 9. The radio tag of claim 1, furthercomprising: a processor coupled to the first receiver, the secondreceiver, and the transmitter, the processor operable to generateresponse signals for transmission in response to the request signalsreceived by the first receiver and the second receiver.
 10. The radiotag of claim 1, further comprising a local power supply.
 11. The radiotag of claim 2, wherein the second receiver comprises: a localoscillator operable, when active, to generate a demodulated signal bydemodulating received traffic; a detector coupled to the localoscillator to detect communicated signals in the demodulated signal; anda quench oscillator coupled to the local oscillator and operable toperiodically activate the local oscillator for the purpose of detectingthe presence of a communicated signal and to continuously activate thelocal oscillator in response to the presence of the communicated signalfor the purpose of collecting the communicated signal.
 12. The radio tagof claim 11, wherein the local oscillator is operable to demodulatereceived traffic at the first frequency and the communicated signal istraffic modulated at the first frequency.
 13. The radio tag of claim 11,the quench oscillator comprising: a first stage quench oscillatorcoupled to the local oscillator and operable, when active, to activatethe local oscillator; and a second stage quench oscillator coupled tothe first stage quench oscillator and operable in a low power mode toperiodically activate the first stage quench oscillator in order toperiodically activate the local oscillator for the purpose of detectingthe presence of the communicated signal and operable in a full powermode to continuously activate the first stage quench oscillator in orderto continuously activate the local oscillator for the purpose ofcollecting the communicated signal.
 14. The radio tag of claim 13,further comprising a microcontroller coupled to the detector and thequench oscillator and operable to control the mode of the second stagequench oscillator in response to detection of the communicated signal bythe detector.
 15. The radio tag of claim 14, wherein the microcontrolleris operable to automatically transition the second stage quenchoscillator from the low to the full power mode in response to detectionof the communicated signal by the detector and to automatically returnthe second stage quench oscillator from the full to the low power modefollowing collection of the communicated signal.
 16. The radio tag ofclaim 13, wherein the low power mode has a limited duty cycle.
 17. Theradio tag of claim 13, wherein the low power mode has a duty cycle offive percent or less.
 18. The radio tag of claim 13, wherein the lowpower mode has a duty cycle of approximately one percent or less. 19.The radio tag of claim 11, wherein the quench oscillator is a dualfrequency oscillator having a low power mode to periodically activatethe local oscillator for the purpose of detecting the presence of thecommunicated signal and a full power mode to continuously activate thelocal oscillator for the purpose of collecting the communicated signal.20. The radio tag of claim 11, further comprising: a battery powersupply; a low power mode for the quench oscillator to periodicallyactivate the local oscillator for the purpose of detecting the presenceof the communicated signal; and wherein the low power mode is powered byonly leakage current from the battery.
 21. The radio tag of claim 20,wherein the leakage current is less than 500 nanoamps.
 22. The radio tagof claim 2, wherein the first receiver comprises: a threshold voltagegenerator coupled to a local power supply and operable to generate athreshold voltage signal on a threshold voltage generator output; and acomparator having a first comparator input coupled to the antenna systemto accept a received signal and a second comparator input coupled to thethreshold voltage generator output to receive the threshold voltagesignal, the comparator powered by the local power supply and operable tocompare the received signal to the threshold voltage signal and togenerate a digital output based on the comparison.
 23. The radio tag ofclaim 22, wherein the comparator and the threshold voltage generator arepowered by only leakage current from the local power supply.
 24. Theradio tag of claim 22, wherein the comparator and the threshold voltagegenerator are powered by less than four microamps of current from thelocal power supply.
 25. The radio tag of claim 22, wherein a powerconsumption of the comparator is approximately three microamps or less.26. The radio tag of claim 22, wherein a power consumption of thethreshold voltage generator is less than 1 microamp of current from thelocal power supply.
 27. The radio tag of claim 22, wherein the localpower supply is a battery power supply.
 28. The radio tag of claim 22,wherein the comparator has a propagation delay of less than fifteenpercent of a period of a carrier signal of the second frequency on whichcommunicated signals are received such that at least seven digitaloutputs are generated for each period.
 29. The radio tag of claim 22,wherein the comparator has a propagation delay of approximately tenpercent of a period of a carrier signal of the second frequency on whichcommunicated signals are received such that at least ten digital outputsare generated for the received signal during each period.
 30. The radiotag of claim 22, wherein the comparator has a propagation delay of lessthan one microsecond.
 31. The radio tag of claim 22, the thresholdvoltage generator comprising: a first resistor coupled to the localpower supply; a second resistor coupled in series to the first resistor,the second resistor further coupled to a ground; and the thresholdvoltage generator output coupled to a connection between the firstresistor and the second resistor.
 32. The radio tag of claim 31, furthercomprising: a capacitor coupled to the connection between the firstresistor and the second resistor, the capacitor further coupled to theground, the capacitor operable to maintain a substantially constantvoltage on the threshold voltage generator output.
 33. The radio tag ofclaim 22, wherein the threshold voltage signal is less than 50millivolts.
 34. The radio tag of claim 22, wherein the threshold voltagesignal is less than 500 millivolts.
 35. A radio tag, comprising: anantenna system; a first receiver coupled to the antenna system andoperable to receive and process signals from a first unit type at afirst frequency, the first receiver comprising: a local oscillatoroperable, when active, to generate a demodulated signal by demodulatingreceived traffic; a detector coupled to the local oscillator to detectcommunicated signals in the demodulated signal; and a quench oscillatorcoupled to the local oscillator and operable to periodically activatethe local oscillator for the purpose of detecting the presence of acommunicated signal and to continuously activate the local oscillator inresponse to the presence of the communicated signal for the purpose ofcollecting the communicated signal; a second receiver coupled to theantenna system and operable to receive and process request signals froma second unit type at a disparate second frequency, the second receivercomprising: a threshold voltage generator coupled to a local powersupply and operable to generate a threshold voltage signal on athreshold voltage generator output; and a comparator having a firstcomparator input coupled to an antenna to accept a received signal and asecond comparator input coupled to the threshold voltage generatoroutput to receive the threshold voltage signal, the comparator poweredby the local power supply and operable to compare the received signal tothe threshold voltage signal and to generate a digital output based onthe comparison; a transmitter coupled to the antenna system and operableto transmit response signals at the second frequency.
 36. The radio tagof claim 35, wherein the second unit type has a limited power supply andthe first frequency is a lower frequency than the second disparatefrequency.
 37. A method for communications in a radio frequencyidentification system comprising: transmitting base station messages toportable radio frequency tags at a first frequency; and transmitting tagmessages to the base station at a second disparate frequency.
 38. Themethod of claim 37, further comprising transmitting inter tag messagesat the second frequency.
 39. The method of claim 37, wherein the firstfrequency is a lower frequency than the second frequency.
 40. The methodof claim 37, wherein the first frequency is a very low frequency (VLF)and the second frequency is an ultra high frequency (UHF).